This invention relates to AGC amplifiers and more particularly to AGC circuits capable of fast response time. The invention is particularly useful in demodulating data from multiple track magnetic disk video recording systems where a complete entire video field comprising a plurality of lines separated by vertical blanking intervals is recorded on a track and one or more heads progress incrementally from one track to another for each field.
In such applications, the head is moved to a new track in sequence or randomly and begins to output a data signal at some level. The AGC demodulator circuits have to adjust gain automatically so as to provide a constant level output signal. The time available for adjusting the gain and recovering gain control after moving the head between fields of the signal and after a video blank pulse between the lines of each field in video disk recording applications is so small as to require AGC response in as little as one to five microseconds. Such requirements are not been met by currently available circuits. There is therefore a need for an AGC amplifier which is capable of reliable fast response in the above mentioned circuits.